1. Field of the Invention:
This invention relates to recirculating hydraulic systems in which a pump supplies pressure for the operation of a separately located hydraulic device. It is useful for the operation of hydraulic devices on construction machinery and the like although it is not limited to such usage.
2. Description of the Prior Art:
It is known that air is a contaminant in the hydraulic fluid for such systems, just as dirt, water and acid are. For an indepth discussion of this topic see the article entitled "Keeping Air Out of Hydraulic Systems" by Vincent G. Magorien in the Aug. 7, 1980 issue of Machine Design.
In Table 6 in the article the author shows vacuuming as one method of removing entrained and dissolved air from hydraulic fluid. However, it is not necessary to subject the hydraulic fluid to a vacuum to remove the air. By continually varying the hydraulic fluid pressure in the system while such pressure is always above atmospheric pressure it is possible to remove sufficient air from the hydraulic fluid to in large measure nullify its deleterious effects.
Our hydraulic system is similar to that disclosed in U.S. Pat. No. 4,052,852 Hart except that our system utilizes a continually varying super atmospheric pressure instead of the constant positive pressure of the Hart system.
An article in the BFPR Journal 1979, 12, 1, 59-63 of the Basic Fluid Power Research Program, Oklahoma State University, on page 62 discusses the use of low pressure start-up conditions to promote deaeration and the release of air from the system at the time of start-up. This invention provides for release of air from the hydraulic fluid during operation of the system and the discharge of such air from the system at the next start-up.